Utilizing the technique of high resolution selective field ionization it is possible to observe ionization structure as narrow as 2 V/cm for Na(nS,nD) atoms. Field ionization structure this narrow is characteristic of atoms evolving predominantly adiabatically in a rapidly increasing electric field. By increasing the slew rate of the applied field it is possible to induce departures from the adiabatic path at specific avoided level crossings. These departures result in multiple ionization thresholds which may be explained by arguments based on a simple quantum mechanical model of adiabatic field ionization.